This invention relates to compositions and methods used in reducing the viscosity of surfactant gels, especially for use in treatment of subterranean formations and oil and gas wells.
Viscoelastic surfactant gels normally are made by mixing appropriate amounts of suitable surfactants, such as anionic, cationic, nonionic and zwitterionic surfactants. The viscosity of viscoelastic surfactant gels has been attributed to the three dimensional structure formed by these components when mixed. When the concentration of surfactants in a viscoelastic fluid significantly exceeds a critical concentration, and in most cases in the presence of an electrolyte, surfactant molecules aggregate into species such as micelles, which can interact to form a network exhibiting elastic behavior. As used herein, the term “micelle” is defined to include any structure that minimizes the contact between the lyophobic (“solvent-repelling”) portion of a surfactant molecule and the solvent, for example, by aggregating the surfactant molecules into structures such as spheres, cylinders, or sheets, wherein the lyophobic portions are on the interior of the aggregate structure and the lyophilic (“solvent-attracting”) portions are on the exterior of the structure. These micelles may function, among other purposes, to stabilize emulsions, break emulsions, stabilize a foam, change the wetability of a surface, solubilize certain materials, and/or reduce surface tension. When used as a gelling agent, the molecules (or ions) of the surfactants used associate to form micelles of a certain micellar structure (e.g., rodlike, wormlike, vesicles, etc., which are referred to herein as “viscosifying micelles”) that, under certain conditions (e.g., concentration, ionic strength of the fluid, etc.) are capable of, inter alia, imparting increased viscosity to a particular fluid and/or forming a gel. Certain viscosifying micelles may impart increased viscosity to a fluid such that the fluid exhibits viscoelastic behavior (e.g., shear thinning properties) due, at least in part, to the association of the surfactant molecules contained therein. As used herein, the term “surfactant gel” refers to a fluid[[s]] that exhibits or is capable of exhibiting viscoelastic behavior due, at least in part, to the association of surfactant molecules contained therein to form viscosifying micelles.
After the surfactant gel has performed its desired function, it is oftentimes desirable to “break” the gel, i.e., reduce its viscosity. Currently, surfactant gels rely upon two methods of breaking: dilution with formation fluids and chemical breakers. Dilution with formation fluids is an unreliable method. For example, in oilfield applications, the viscosity of viscoelastic surfactant gels may be reduced or lost upon exposure to formation fluids (e.g., crude oil, condensate and/or water); and this viscosity reduction or loss effectuates cleanup of the reservoir, fracture, or other treated area. However, in some circumstances, it is desirable to have a better control of that breaking, for instance, when breaking of the fluid is desired at a particular time or condition, when it is desired to accelerate viscosity reduction, or when the natural influx of reservoir fluids (for example, in dry gas reservoirs) does not break or breaks incompletely the viscoelastic surfactant gel. Using chemical breakers can be complicated. Various types of alcohols, organic acids, enzymes, transition metals (e.g., iron), and salts are known to impart a reduction of the viscosity of a viscoelastic gel or even to completely “break” the gel. Such chemical breakers may be added to a pad or a pre-pad fluid, or they may be used in such a way as the mechanism relies upon melting, slow dissolution of a solid, de-adsorption of a breaking agent absorbed into a solid particle, or the breaking of a coating (encapsulated breaker). Oftentimes, such breakers remain in the fractures in the subterranean formation, and as a result, do not contact the gel to a sufficient extent to adequately break the gel. Moreover, if the breaker is a solid breaker, the breaker is likely to settle out of the gel, which can result in either an inefficient or premature break of the surfactant gel.
Therefore, among other needs, needs may exist that may relate to methods for breaking viscoelastic surfactant gels after subterranean oil or gas well treatments, at predetermined times or conditions and/or when they are not broken by the natural influx of reservoir fluids.